A common budgeting mistake is treating software as a one-time purchase rather than an ongoing cost. Whether you're paying a SaaS subscription or commissioning custom development, the sticker price at launch is only part of the real cost over the system's lifetime.
For custom software or websites, budget explicitly for maintenance — security updates, bug fixes, hosting, and small enhancements as business needs shift. A reasonable rule of thumb used across the industry is to expect ongoing annual maintenance in the range of 15–20% of the original build cost, though this varies by project complexity. Businesses that skip this and assume a system is "done" at launch are often the ones running years-old, unpatched software that eventually becomes a security or reliability liability.
For subscription software, model costs at your expected scale, not your current one — many SaaS pricing tiers scale with users, transactions or data volume, and a tool that looks affordable today can become expensive as you grow into it. It's worth asking a vendor directly what the cost looks like at 2x and 5x your current usage before committing.
Finally, budget a contingency for the unplanned: integrations that turn out to be more involved than expected, or a requirement that only becomes clear once staff start using a new system day to day. Projects that budget zero room for this discover the cost anyway — just as an unplanned overrun instead of a planned buffer.